This application is based on applications Nos. 2000-159043, 2000-161267, 2000-194395, 2000-198451, 2000-227640, 2000-258027 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a photoelectric conversion device, in particular, to a photoelectric device using numerous crystalline semiconductor particles. The photoelectric conversion device according to this invention is utilized suitably in solar cells.
2. Description of the Related Art
Advent of a next-generation, low-cost solar cell that allows the quantity of silicon material to be small has been eagerly awaited.
Conventional photoelectric devices in which granular or spherical silicon crystal grains are used are shown in FIGS. 14-17.
The photoelectric conversion device in FIG. 14 comprises a low melting-point metal (tin) layer 108 formed on a substrate 101, numerous crystalline semiconductor particles 103 of a first conductivity-type deposited on the low melting-point metal layer 108, and an amorphous semiconductor layer 107 of a second conductivity-type formed on the crystalline semiconductor particles 103 (Refer to Japanese Patent No. 2641800).
In addition, an insulating layer 102 (SiO2) is interposed between the amorphous semiconductor layer 107 and the low melting-point metal layer 108.
Since the amorphous semiconductor layer 107 serves as the second conductivity-type semiconductor layer in the photoelectric device in FIG. 14, the thickness of the amorphous semiconductor layer 107 needs to be small taking the great light absorption thereof into account. For this reason, the film thickness locally varies when the amorphous semiconductor layer 107 is formed along the surfaces of the crystalline semiconductor particles 103. In general, the film thickness in the part on the crystalline semiconductor particles 103 is large, and that in the side of the crystalline semiconductor particles 103 is small.
The thin films fail to sufficiently cover the whole surfaces of the crystalline semiconductor particles 103 making it difficult to form a PN-junction along the outer contours of the crystalline semiconductor particles 103.
Also, the small thickness of the amorphous semiconductor layer 107 makes the tolerance to defects small also necessitating stricter management of the cleaning process and the production environment.
In addition, since the thin amorphous semiconductor layer 107 has high resistance, a transparent conductive film needs to be formed as the upper electrode on the amorphous semiconductor layer 107, which results in a high manufacturing cost.
In this device, since the insulating layer 102 is formed after securing the particles 103 on the low melting-point metal layer 108, the insulating layer 102 is formed not only on the low melting-point metal layer 108 but also on the particles 103. Accordingly, the insulating layer 102 on the particles 103 needs to be removed before the formation of the amorphous semiconductor layer 107.
Although it is possible to form the amorphous semiconductor layer 107 after grinding the crystalline semiconductor particles 103 and the insulating layer 102 so as to be exposed in a flat surface, this necessitates addition of a grinding process and a cleaning process for removing chips after the grinding. Furthermore, when there is unevenness in the heights of crystalline semiconductor particles 103, the PN-junction area also locally varies causing insufficient properties. As a result, the device suffers high costs and low conversion efficiency.
Also, the low melting point of the low melting-point metal layer 108 makes its reliability low.
Regarding the auxiliary electrode, the Japanese Patent No. 2641800 only describes that an appropriate metal collector electrode is formed, and it includes no specific description about preferred electrodes.
FIG. 15 discloses a photoelectric conversion device in which an upper aluminum foil 111 is formed with openings 111a with which silicon balls 110 are contacted. Each of the silicon balls 110 has a n-type surface portion 110b formed on a p-type core 110a. The n-type surface portions 110b in the rear surfaces of the silicon balls 110 are removed, and the aluminum foil 111 is coated with an oxide layer 114. The oxide layer 114 in the rear surfaces of the silicon balls 110 are removed so that the p-type cores 110a are contacted with a lower aluminum foil 113 (Japanese Unexamined Patent publication S61-124179(A)).
The device in FIG. 15 requires production of the silicon balls 110 having the n-type surface portions 110b formed on the p-type cores 110a. In addition, the openings 111a need to be formed in the aluminum foil 111 so that the silicon balls 110 are pressed into and contacted with the openings 111a. This requires the silicon balls 110 to be uniform in diameter, resulting in high cost manufacture.
The photoelectric conversion device in FIG. 15 has another problem in that when the silicon balls 110 are contacted with the lower aluminum foil 113, the lower aluminum foil 113, the substrate, melts when the junction temperature increases to no less than 577xc2x0 C., which is the eutectic temperature of aluminum and silicon. As a result, the silicon balls 110 penetrate the aluminum foil 113.
FIG. 16 discloses a method in which semiconductor microcrystalline grains 123 are deposited on a substrate 120, and then fused, saturated and gradually cooled so that the semiconductor is grown by liquid phase epitaxial growth, thereby forming a first conductivity-type polycrystalline thin film 123 (Japanese Patent Publication No.H8-34177(B)).
FIG. 16 shows a low melting-point metal film 121 comprising a metal such as Sn, a high melting-point metal film 122 comprising a metal such as Mo, a second conductivity-type polycrystalline or amorphous semiconductor layer 124, and a transparent conductive film 125.
In the photoelectric conversion device in FIG. 16, the low melting-point metal 121 mixes into the first conductivity-type liquid phase epitaxial polycrystalline layer 123, thereby deteriorating the performance of the polycrystalline layer 123. In addition, due to the absence of an insulator between the transparent conductive film 125 and the high melting-point metal film 122, short circuit is likely to occur.
FIG. 17 discloses a photoelectric conversion device, wherein an aluminum film 132 is formed on the surfaces of a steel substrate 131, the aluminum film 132 being contacted with crushed silicon particles 133, and then an insulating layer 136, n-type silicon portions 134, and a transparent conductive layer 135 are sequentially formed (U.S. Pat. No. 4,514,580).
In this photoelectric device, since the aluminum film 132 is formed on the surface of the steel substrate 131, the aluminum film 132 is oxidized first, and the steel substrate 131 is oxidized thereafter. This makes the reliability of this photoelectric conversion device low, and the life thereof short.
In order to secure the reliability, the aluminum film 132 needs to be thickened. However, when the aluminum film 132 is thickened, the crushed silicon particles 133 in contact with it need to have diameters increased accordingly, which is a problem.
There is a description in the disclosure of the photoelectric conversion device in FIG. 17 meaning that the electrode may comprise any desired pattern of bus bars and fingers. However, it has no specific description of preferred auxiliary electrodes.
It is an object of this invention to provide a photoelectric conversion device with excellent properties.
A photoelectric conversion device according to the present invention comprises: first conductivity-type crystalline semiconductor particles being deposited on and in contact with a substrate in a large number; an insulator formed in an area on the substrate where the crystalline semiconductor particles are not present; and a second conductivity-type semiconductor layer formed over the crystalline semiconductor particles and the insulator so as to form a PN-junction between the semiconductor layer and the crystalline semiconductor particles, wherein the second conductivity-type semiconductor layer comprises a semiconductor layer including a crystalline semiconductor and an amorphous semiconductor in a mixed manner.
Because layer-depositing conditions for forming the semiconductor layer including a crystalline semiconductor and an amorphous semiconductor in a mixed manner are wider than the conditions for forming a crystalline semiconductor layer, this photoelectric conversion device yields a large manufacturing margin, thereby allowing for a low-cost production.
Also, the particle diameters of the crystalline semiconductor particles having a single conductivity type do not need to have a high level of uniformity, which enables the device to be manufactured at low cost.
The second conductivity-type semiconductor layer comprising a crystalline semiconductor and an amorphous semiconductor in a mixed manner has high light transmittance, which allows the thickness thereof to be more flexible so that the layer exhibits good covering performance.
The insulator ensures separation of the positive electrode and the negative electrode, thereby preventing short circuit from occurring between the crystalline semiconductor particles and the conductive film. The device therefore has high conversion efficiency.
Accordingly, when compared with known photoelectric conversion devices, the device according to this invention enables the manufacture thereof to be carried out at lower cost while exhibiting high properties.
The aforementioned second conductivity-type semiconductor layer can function also as the upper electrode. Accordingly, boring a hole in the electrode, which is necessary when the upper electrode is separately provided, is no longer necessary in this device. This permits utilization of the light incident on the area without semiconductor particles. As a result, the device can be more inexpensive and achieve better conversion efficiency.
Furthermore, owing to the flexibility in shape, the photoelectric conversion device of this invention has little dependence on the light incidence angle.
A photoelectric conversion device according to the present invention comprises: first conductivity-type crystalline semiconductor particles being deposited on and in contact with a substrate in a large number; an insulator formed in an area on the substrate where the crystalline semiconductor particles are not present; and a second conductivity-type semiconductor layer formed over the crystalline semiconductor particles and the insulator so as to form a PN-junction between the semiconductor layer and the crystalline semiconductor particles, wherein the substrate is formed by laminating a first aluminum-iron alloy layer, an iron layer, a second aluminum-iron alloy layer, and an aluminum layer.
Preferably, the above-mentioned substrate should be formed by sequentially laminating and heating a first aluminum layer, an iron layer, and a second aluminum layer.
By providing aluminum having a low melting point and large electric conductivity in the area where the substrate and the crystalline semiconductor particles are in contact, low-temperature junction and low-resistance electrode can be realized in this photoelectric conversion device.
However, when the substrate consists of only aluminum, fusion-bonding between the crystalline semiconductor particles and the aluminum proceeds without limitation, thereby making control of the junction difficult. As an effective measure for controlling the junction, a metal having a melting point higher than aluminum is disposed beneath the aluminum so that the fusion-bonding does not advance beyond the thickness of the aluminum.
As the metal with a high melting point, iron is optimal in terms of melting point, rigidity, thermal expansivity, and cost. However, iron has a problem of weak weather resistance due to its susceptibility to oxidization.
Accordingly, in this invention, the iron is protected by the aluminum-iron alloy, thereby attaining high reliability.
A photoelectric conversion device according to the present invention comprises: first conductivity-type crystalline semiconductor particles being deposited on and in contact with a substrate in a large number; an insulator formed in an area on the substrate where the crystalline semiconductor particles are not present; and a second conductivity-type semiconductor layer formed over the crystalline semiconductor particles and the insulator so as to form a PN-junction between the semiconductor layer and the crystalline semiconductor particles, wherein the relation in weight between the insulator and the crystalline semiconductor particles is V1xc3x97xcfx811xe2x89xa7V2xc3x97xcfx812 (V1: volume of one crystalline semiconductor particle, V2: volume of one crystalline semiconductor particle in the part being buried in the insulator, xcfx811: specific gravity of the crystalline semiconductor particles, xcfx812: specific gravity of the insulator).
This photoelectric conversion device allows for formation of a good insulator while a junction is formed between the substrate and the semiconductor particles, thereby ensuring separation of the positive electrode and the negative electrode. As a result, this device provides larger manufacturing margin and lower manufacturing cost when compared with conventional photoelectric conversion devices.
A photoelectric conversion device according to the present invention comprises: first conductivity-type crystalline semiconductor particles being deposited on and in contact with a substrate in a large number; an insulator formed in an area on the substrate where the crystalline semiconductor particles are not present; a second conductivity-type semiconductor layer formed over the crystalline semiconductor particles and the insulator so as to form a PN-junction between the semiconductor layer and the crystalline semiconductor particles; and an electrode formed in the second conductivity-type semiconductor layer, wherein the area of a portion of the electrode formed in the second conductivity-type semiconductor layer that is present above the crystalline semiconductor particles is 10% or less of the whole area of the electrode.
According to this photoelectric device, the electrode is formed such that the area of the electrode under which the crystalline semiconductor particles are present is only 10% or less of the whole area of the electrode.
This allows the photoelectric conversion device to have high reliability as well as high properties.
A photoelectric conversion device according to the present invention comprises: first conductivity-type crystalline semiconductor particles being deposited on and in contact with a substrate in a large number; an insulator formed in an area on the substrate where the crystalline semiconductor particles are not present; and a second conductivity-type semiconductor layer formed over the crystalline semiconductor particles and the insulator so as to form a PN-junction between the semiconductor layer and the crystalline semiconductor particles, wherein the second conductivity-type semiconductor layer comprises two layers each of which has an impurity addition concentration that differs from the other such that the impurity addition concentration in the lower layer of the second conductivity-type semiconductor layer is lower than that in the upper layer of the second conductivity-type semiconductor layer.
According to this photoelectric conversion device, the upper layer of the second conductivity-type semiconductor layer with higher impurity addition concentration can reduce the series resistance and prevent the conversion efficiency from lowering.
In addition, due to the lower impurity addition concentration in the lower layer of the second conductivity-type semiconductor layer, leakage current is prevented from occurring. Accordingly, this device can achieve conversion efficiency higher than that of known photoelectric conversion devices.